It is known that most of the isolates of B.thuringiensis show a toxic activity with regard to larvae of more than a hundred species of Lepidoptera.
This activity results from the capacity of the strains of B.thuringiensis to synthesize, at the moment of sporulation, crystalline inclusions of protein nature, or .delta.-endotoxins, under the control of one or several types of gene.
It has been shown that the activity of these polypeptides is contained in the NH.sub.2 -terminal half or N-terminus of the protein.
The studies carried out have shown the high specificity of the .delta.-endotoxins towards larvae of a given species.
On account of this high specificity, many species of Lepidoptera, in particular of the family of the Noctuidae, react only weakly to commercial preparations of available B.thuringiensis.
It is so in particular for the species S.littoralis, a polyphagous insect which constitutes the principal parasite of cotton and other industrially important crops. Among these crops, mention should be made of maile, the castor oil plant, tobacco, the groundnut, fodder plants, such as clover or alfalfa, or also market garden produced such as the cabbage or the tomato.
Hence, one can imagine the interest of disposing of agents targeting specifically and effectively the family of the Noctuidae and in particular S.littoralis or M.brassicae.
The genes for .delta.-endotoxins hitherto identified do not code for a polypeptide preferentially active with regard to S.littoralis.